1. Field of the Invention
The present invention relates to a lithium secondary battery, and more particularly, to a pouch-type lithium secondary battery with improved arrangement of an electrode tab with respect to the sealing surface of a case thereof.
2. Description of the Related Art
Recently, with development of portable electronic devices such as cellular phones, notebook computers, and camcorders, chargeable/dischargeable secondary batteries have been actively studied. Examples of such secondary batteries include nickel-cadmium batteries, nickel-metal hydride batteries, nickel-hydrogen batteries, and lithium secondary batteries. Among these batteries, lithium secondary batteries have an operation voltage of 3.6 V, which is three times higher than the operation voltage of nickel-cadmium batteries or nickel-metal hydride batteries, which have been the main sources of power for portable electronic devices. In addition, lithium secondary batteries have an excellent energy density per unit weight. Therefore, use of lithium secondary batteries has been rapidly increasing.
According to the kind of an electrolyte used, lithium secondary batteries are classified as lithium ion batteries, which use a liquid electrolyte, and lithium polymer batteries, which use a polymer electrolyte.
Lithium secondary batteries can be manufactured in various shapes. Representative shapes of lithium secondary batteries are a cylinder and a prism, which are common shapes of lithium ion batteries. Recently, a flexible pouch-type lithium polymer battery has been attracting attention because of its relatively easily changeable shape, excellent safety, and lightweight. Such a pouch-type lithium polymer battery enables further reduction in the size and weight of portable electronic devices.
FIG. 1 illustrates a top plan view of a conventional lithium secondary battery.
Referring to FIG. 1, the lithium secondary battery 10 comprises a battery unit 11 and a case 12, which is formed with a space 12a for receiving the battery unit 11.
The battery unit 11 comprises a positive plate, a negative plate, and a separator interposed therebetween. The battery unit 11 is formed by winding the positive plate, the separator, and the negative plate, which are sequentially wound into a jelly-roll structure, or sequentially laminating the positive plate, the separator, and the negative plate into a stack structure.
A positive tab 13 and a negative tab 14 electrically connected to the respective electrode plates of the battery unit 11 are installed such that a predetermined length of them is exposed outside upper and lower sealing surfaces 12b of the case 12. Portions of the electrode tabs 13 and 14 that come in contact with the upper and lower sealing surfaces 12b are wrapped with an insulating tape 15.
As illustrated in FIG. 3, unlike a cylindrical or prismatic can made of a metal material, the case 12 has a pouch-type structure, which comprises an intermediate layer 12d made of a metal foil, and inner and outer layers 12e and 12f made of an insulating film. The case 12 is formed with the space 12a for receiving the battery unit 11 and with the upper and lower sealing surfaces 12b, which are thermally fused to each other.
FIG. 2 illustrates a sectional view of the pouch-type lithium secondary battery of FIG. 1.
Referring to FIG. 2, portions of the electrode tabs 13 and 14 are positioned between the upper and lower sealing surfaces 12b of the case 12 and end portions of the electrode tabs 13 and 14 are exposed outside the upper and lower sealing surfaces 12b of the case 12. Portions of the electrode tabs 13 and 14, which come in contact with the upper and lower sealing surfaces 12b, are wrapped with an insulating tape 15. In order to prevent the inner layer 12e from being pushed outward when the upper and lower sealing surfaces 12b are thermally fused to each other, end portions 12c of the upper and lower sealing surfaces 12b are upwardly and downwardly inclined at a predetermined angle with respect to the electrode tabs 13 and 14.
In order to manufacture the lithium secondary battery 10 with the aforementioned structure, first, the battery unit 11 is formed by winding or laminating a positive plate and a negative plate with a separator interposed therebetween. The formed battery unit 11 is inserted into the space 12a of the case 12. Then, the upper and lower sealing surfaces 12b of the case 12 are thermally fused to each other to hermetically seal the battery unit 11. In this case, the insulating tape 15, which wraps around the electrode tabs 13 and 14 and is interposed between the upper and lower sealing surfaces 12b, is also thermally fused to the upper and lower sealing surfaces 12b. 
Next, end portions of the electrode tabs 13 and 14, which are exposed outside the case 12, are folded toward the case 12 at least one time. The end portions of the folded electrode tabs 13 and 14 are positioned outside one of the upper and lower sealing surfaces and are electrically connected to the terminal of a separately prepared protective circuit board (not shown).
However, such a conventional lithium secondary battery 10 has the following problems.
When the electrode tabs 13 and 14 are folded toward the case 12 to electrically connect the end portions of the electrode tabs 13 and 14 to the protective circuit board, the inner surfaces of the electrode tabs 13 and 14 come in contact with the end portion 12c of the upper or lower sealing surface 12b. In this case, the intermediate layer 12d of the end portion 12c, which is exposed outside by a cutting process for completing the shape of the case 12, comes in contact with the electrode tabs 13 and 14. As a result, a short circuit between the case and the electrode tabs may occur. In addition, the protective circuit board, which is positioned outside one of the upper and lower sealing surfaces 12b, is likely to come in direct contact with outer surfaces of the upper and lower sealing surfaces 12b. For these reasons, the safety and reliability of the lithium secondary battery 10 cannot be assured.